1. Field of the Invention
The present invention relates to EGR and combustion control processes to assist in the cost-effective lowering of harmful NOx emissions produced in diesel internal combustion engines, with good transient response times.
2. Background of the Invention, and Description of the Related Art
The continuing use of diesel engines in motor vehicles greatly adds to the atmospheric presence of harmful pollutants such as nitrogen oxides (NOx). Conventional diesel engines emit NOx substantially in excess of acceptable environmental levels. Nevertheless, because of their fuel efficiency, diesel engines remain preferable to gasoline engines for many applications. Attempts to reduce NOx emissions from diesel engines have therefore continued for many years.
There is a need in the art for a cost-effective robust diesel combustion system that is capable of maintaining emissions levels of NOx within upcoming federally mandated environmentally acceptable levels (defined for purposes of this invention as 0.2 g/bhp-hr or lower, after possible exhaust aftertreatment). Reducing NOx emissions in diesel engines to the 0.2 g/bhp-hr standard is generally believed to require utilization of NOx aftertreatment such as a NOx absorber, NOx trap, or Urea/SCR. However, industry views such NOx aftertreatment technologies as presenting cost and durability challenges. It is therefore potentially desirable to use, in conjunction with and in addition to such NOx aftertreatment, methods to reduce NOx formation in combustion and thereby reduce the NOx-reduction burden to be handled through NOx aftertreatment.
Exhaust gas recirculation (EGR) is a known method in the art to lower engine-out NOx emissions, although the extent of EGR use is generally limited by an increase in smoke formation and loss of efficiency that occurs as the EGR ratio reaches high levels. EGR usage to lower NOx formation in diesel engines has generally stayed at EGR/ambient air ratios lower than 25% for medium and higher load conditions.
High pressure EGR (i.e., recirculating exhaust gas upstream of the turbine in the exhaust line, before the exhaust gas expands in the turbine) is the most common form of exhaust gas recirculation. However, significant use of high pressure EGR requires significant and expensive cooling of the hot recirculated exhaust gas, as well as an adverse pressure differential across the engine to move the EGR from the exhaust to the engine intake system.
Low pressure loop EGR systems are an attractive alternative that may also be used to lower engine-out NOx emissions, allowing some natural cooling of the exhaust gas (e.g., in expansion and transport of the exhaust gas through the turbine and exhaust line) and therefore reducing cooling requirements and costs for EGR, as well as avoiding the pumping losses inherent in high pressure EGR systems. But low pressure EGR systems are nevertheless generally avoided in the art due to concerns with transport delays (lag time) in EGR adjustments associated with such systems, which are made by adjustment of EGR levels can generally pace the system's response time during transients. Attempts to control in-cylinder NOx formation through low pressure EGR valve adjustments are challenged to keep up with the transient response times and system responsiveness desired for motor vehicle engine applications.
As a result, in-cylinder control of NOx formation during transients has been difficult to achieve in the art. Transient changes in the operating conditions of a diesel engine, such as upon vehicle acceleration, can result in significant NOx or PM emissions if EGR flow rate and level adjustments do not keep up with the changes in fuel feed and boost levels necessary to meet the change in power demand. For example, temporary fuel levels in excess of desired fuel/oxygen ratios can occur in transients, with resulting increased PM levels. Likewise, if intake oxygen concentrations rise in transients, an increase in NOx emissions will result. Such problems are a common occurrence in the prior art, as it is also conventional to stop exhaust gas recirculation during an increase power demand, which therefore results in an increase in the intake oxygen concentration, and can therefore cause spikes in NOx emissions during such transients. Such emissions during transient changes can cause vehicles to fail emission standards even where such vehicles could meet the emission standards at steady state conditions.
Thus, it is desirable to reduce the time required for EGR adjustments during transients in a low pressure EGR loop system, when the goal is to maintain consistent or low NOx emissions during rapid engine speed and load transitions.
As disclosed in commonly assigned U.S. Pat. No. 6,857,263, the teachings of which are incorporated herein by reference, active control of the oxygen concentration of charge-air used in combustion in a diesel engine can be used to significantly reduce formation of NOx in combustion. Said commonly assigned U.S. Pat. No. 6,857,263 also discloses one method for maintaining control of the oxygen concentration of charge-air used in combustion in a diesel engine through transients without an impediment to engine responsiveness, by maintaining the exhaust oxygen concentration and intake oxygen concentration relatively constant from cycle to cycle for at least medium and high loads, and through transients. The relatively constant oxygen concentration of charge-air used for combustion could be any range selected below 20%, depending on the particular NOx formation acceptable for the engine system. Without a NOx aftertreatment system, the oxygen concentration is preferably below 16%, within a tight range somewhere between 10% and 15%. On the other hand, with a NOx aftertreatment device present in the system, the oxygen concentration range for combustion could be higher for better efficiency (e.g. 16%, 17%, 18%, or 19%). The control of oxygen concentration to a relative constant allows continued control of NOx formation through transients without a need for transient response to be paced by adjustments in the low pressure EGR system.
However, for improved efficiency, it would also be desirable if the oxygen concentration of the charge-air used in combustion could be selectively controlled to an oxygen concentration varying with engine condition, with good transient response.